This disclosure sets forth a cold, field emission device, wherein low noise microampere electron currents are achieved at low fields, typically 5.times.10.sup.8 volts/meter, from applied potentials of typically 4.85 volts, commonly available at a nominal value of 5 volts. Emission is across the less than 1 ev barrier from the conduction band of a multiplicity of insulator particles in ohmic contact with conductor particles. The multiplicity of insulator particles emits in a stochastic manner, increasing the current-plus-noise to noise ratio by at least 20 decibels over the prior art devices.
Exemplary field emission devices are set forth in the patents of Fraser, U.S. Pat. No. 3,735,022 and Spindt, U.S. Pat. No. 3,755,704 which disclosures reveal molybdenum as the emitter material. Copper emitter materials are shown in Levine, U.S. Pat. No. 3,921,022. Redman, U.S. Pat. No. 3,982,147 and Shelton, U.S. Pat. No. 4,163,198 utilize metal fibers. Fukase, et al, U.S. Pat. No. 3,998,678 discloses an emitter of lanthanum hexaboride or other rare earth borides. The patent of Hosoki, U.S. Pat. No. 4,143,292 discloses carbon used as a metal. In all the prior art typical fields of at least 5.times.10.sup.9 volts/meter are applied, to raise sufficient electrons from the metal Fermi level, 2.3 ev to 4.5 ev, to vacuum level. The barrier (a range of 2.3 ev to 4.5 ev) to emission is the range typified by all the prior art including the work function of rare earth borides through tungsten metal. The quotient, the work function to the 3/2 power divided by the applied field, is part of the negative exponent of the exponential expression for the probability of emission. Such high fields, in excess of that required to overcome the surface tension of the material, cause protruberances ("whiskers") to grow from the emitting surface. The enhanced field about a whisker makes the whisker the source of emission. The relatively large geometries and work functions of the prior art require potentials of 50 volts to 2,500 volts to produce the required field to both produce and emit from a whisker. Such high potentials produce electron energies many times that required to ionize gas molecules. This disclosure is directed to emission from a multiplicity of insulative particles having an exemplary reduced 0.85 ev barrier to emission, requiring typically a field of 5.times.10.sup.8 volts/meter obtained from a potential under 5 volts. Such a low field is insufficient to cause the growth of whiskers. Such low potential (e.g., 5 volts) produces electrons with insufficient energy to ionize gasses.
The emitting device volume of all prior art structures is relatively large such that millions of gas molecules are available to collision with emitted electrons, causing beamspreading. Notwithstanding the nanotorr vacuum required in all prior art devices, millions of gas molecules remain and are available to be ionized in the vicinity of field emitters. Such ionized gasses are attracted to and ablate, or adsorb onto, the emitter surface instantaneously changing the work function by changing the surface composition. The changes in work function modify the emitted current. That change, instability, in emitted current is the "burst noise" of all the prior art. The ion ablation of the very small whisker emitting surface results in short operating life. The volume of the structure of the present disclosure accessible to vacuum is very small, such that at 10 microtorr vacuum, not more than 3 gas molecules remain in the total volumetric space. The potentials between elements of the present disclosure device are less than the ionizing potential of residual or diffusing gasses. Thus, the 3 or fewer gas molecules in the volume are neither ionized nor initiate significant collision scattering nor spreading of the beam. The absence of burst noise of the present disclosure device further increases the current to noise ratio as compared to the prior art. The ability of the device of the present disclosure to operate at 10 microtorr vacuum level, as compared to typically 1 nanotorr of prior art devices is an additional economy, reducing the cost of vacuum systems.
In the device of the present disclosure, the energy and velocity of the emitted electrons is low. Such low velocities, together with low lateral energy, make focus and deflection by electrostatic means in micron dimensions feasible. Prior art devices commonly require large electromagnets for focusing the emitted electrons, part of the reason for their large volume. The present disclosure features low velocity electrons highly sensitive to low intensity steering fields. An optional feature to overcome emitted electrons sensitive to ambient magnetic fields and part of the disclosed structure is a high strength, low induction magnet with an aperture centered upon the emitting surfaces. The magnet dominates ambient fields, and produces field lines parallel to and within the emission axis. Those field lines additionally steer ions, created in any exterior acceleration space, away from the emitting surfaces.
The maximum width of the energy distribution of the device of the present disclosure is less than 0.09 ev, about one-tenth that of prior art devices. A narrower distribution of emitted electron energy results in a smaller, better defined beam diameter.
The device of the present disclosure is amenable to manufacture in batches, using process steps also used in the making of semiconductor integrated circuits. All elements of the device, including implicitly self-aligned electron optical elements are fabricated in such an economic process. The prior art devices normally require hand fabricated and assembled focus and deflection elements and are not amenable to the economies of batch manufacturing.